Electro-optic Longitudinal Profile Diagnostics

S P Jamison, Accelerator Science and Technology Centre,

STFC Daresbury Laboratory

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Electro-optic effect for bunch diagnosticsCoulomb field of relativistic bunch

probe laser

encodingof bunch information into

laser

decodingof information

from laser pulse

Measure electric fields of bunch : Coulomb field, CSR, CTR, wakefields, ...

Spectrum of field important

for capability &

technique choice

E(t) Coulomb Field E(w)

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Coulomb spectrum shifted to optical region

Coulomb pulse replicatedin optical pulse

envelope optical field

Electro-optic longitudinal diagnosticsPhysics : Frequency mixing between Coulomb field (or CSR, CTR,FEL …) pulse and probe laser

c (2)(w;wthz,wopt)

wopt + wthzwthz

wopt

wopt - wthz

woptEO cr

ysta

lCoulomb field

probe laser

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Electro-Optic Techniques...

Spectral Decoding

Spatial Encoding

Temporal Decoding

Spectral upconversion**

Variations in read-out of optical temporal signalo Chirped optical input o Spectral readouto Use time-wavelength relationship

o Ultrashort optical inputo Spatial readout (EO crystal)o Use time-space relationship

o Long pulse + ultrashort pulse gateo Spatial readout (cross-correlator crystal)o Use time-space relationship

o monochomatic optical input (long pulse)o Spectral readouto **Implicit time domain information only

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

In general spectral decoding resolution limited by chirp

Spectral DecodingAttractive simplicity for low time resolution measurements e.g. injector diagnostics

Rely on t-l relationship of input pulse for interpreting output optical spectrum Resolution limits come from fact that EO-generated optical field doesn't

have same t-l relationship

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

ALICE Electro-optic experimentso Energy recovery test-accelerator

intratrain diagnostics must be non-invasive

o low charge, high repition rate operation typically 40pC, 81MHz trains for 100us

Spectral decoding results for 40pC buncho confirming compression for FEL commissioningo examine compression and arrival timing along traino demonstrated significant reduction in charge requirements

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Measured EOSDSignal (40pC)

Electro-optic spectral decoding on ALICE

Model bunch-profile

EOSD response fnc.

expected EOSD signal

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Direct Temporal techniques...

Temporal decoding

• Encoding of signal exactly as before..• measure temporal profile of probe pulse directly

using spatial-temporal cross-correlation

envelope optical field

Spatial encoding

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Temporal decoding

Temporal profile of probe pulse Spatial image of 2nd harmonic

o Limited by gate pulse duration … … “frequency resolved optical gating” (FROG) solutions?

o Complex of laser & optical transport systems

o EO interaction produces optical replica of Coulomb fieldo Readout via 2nd Harmonic Generation & optical cross-correlation

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Electro-optic Temporal decodingFLASH, 400 MeV, ~500pC)

65mm thick GaP

Benchmarked against RF deflecting cavity• provides a unique “calibrated” THz source...• confirms understanding of material properties

Berden et al. Phys Rev Lett. 99 (2007)

ALICE, 30 MeV, 60pC

monitoring compression & arrive time (Lattice and beam properties)

Signal-nose issues at low charge

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Time Calibration....

probe laserbunchgate laser

measure the same electron bunch twicewith known measurement time delay

CDR feedback on CDR feedback offConfirmation of feedback systems

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Spectral upconversion diagnosticmeasure the bunch Fourier spectrum...

... accepting loss of phase information & explicit temporal information

... gaining potential for determining information on even shorter structure

... gaining measurement simplicity

Long pulse, narrow bandwidth, probe laser

d-function

NOTE: the long probe is still converted to optical replica

same physics as “standard” EO

different observationaloutcome

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

difference frequency mixing

sum frequency mixing

Spectral upconversion diagnosticFirst demonstration experiments at FELIX

Jamison et al. Applied Physics Letters, 96 231114 (2010)

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

FELIX temporal profile THz spectrum pr

edict

ion

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Current status, future improvementsLow time resolution (>1ps structure)

• spectral decoding offers explicit temporal characterisation• robust laser systems available• diagnostic rep rate only limited by optical cameras

High time resolution (>60 fs rms structure)• proven capability• significant issues with laser complexity / robustness

Very higher time resolution (<60 fs rms structure)• EO material properties (phase matching, GVD, crystal reflection)• Laser pulse duration (TD gate, SE probe)Limited by

Accelerator wish list - Missing capabilities o Higher time resolution (20fs rms for CLIC)o Higher reliability, lower cost (high resolution systems)o solution for feedback.

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Can we achieve even better time resolution ...?

Detector Material:– GaP – Move to new material? ( phase matching, c(2) considerations )– Could use GaSe, DAST, MBANP .....?– use multiple crystals, and reconstruction process

Gate pulse width ~ 50 fs– Introduce shorter pulse– Use (linear) spectral interferometry– Use FROG Measurement (initially attempted at FELIX, 2004)

Encoding

Decoding

or Alternative techniques: spectral upconversion

If drop requirement for explicit time information at high frequencies, other options also become available

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

c (2)(w;wthz,wopt)wopt + wthz

convolution over all combinations of optical

and Coulomb frequencies

Electro-optic detection as sum- and difference-frequency mixing

wthz

wopt

wopt - wthz

wopt

frequency domain description of EO detection...

THz spectrum(complex)

propagation& nonlinear

efficiencygeometrydependent

(repeat for each principle axis)

optical probe

spectrum(complex)

EO c

ryst

al

Refractive index formalism comes out as subset of solutions(restriction on laser parameters)

This is “Small signal” solution. High field effects c.f. Jamison Appl Phys B 91 241 (2008)

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Time resolution & bandwidth

Many variants of EO…

… all involve conversion of Coulomb field “pulse” to optical pulse

• Manageable relative bandwidth• Exploit ultrafast laser diagnostic techniques

CLIC requirements: 20 fs time resolution

Coulomb field …. 0.1 – 20 THz (octave spanning bandwidth) Converted to optical field …. 300 THz +/- 20THz (10% bandwidth

• Implies 20-30 THz detection bandwidth• Uniform (or known) response function

over full bandwidth

Time profile

Spectra

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

200fs

GaP

Effect of Material response...

100fs

100fs 50fs

ZnTe ZnTe

GaP

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Solution in multiple crystals and crystal orientations…

From Shi et al. Appl. Phys. Lett 2004

Tuneable phase matching of laser and THz pulse…

Coulomb spectral component to be

measured…

… crystal angle to achieve phase matching

Questions on how to “splice” data.

• Response amplitude can be measured from detection of tuneable THz source

• Spectral complex response can be measured from THz-TDS from linear THz-TDS … if we have known ultrashort source

GaSe Many candidate crystals

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

time

Cross-correlation method

• Optical probe with electron bunch info• ultrafast “gate” for time->space readout

• Resolution is limited by gate duration (+phase matching)

Practical implementation limits gate to >40fs fwhm ( laser transport, cross-correlator phase matching/signal levels )

• Weak probe due to EO material damage limits…• Compensated by intense gate

Signal/noise issues from this mismatch in intensities

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

frequ

ency

time

• Obtain both time and spectral information• Sub-pulse time resolution retrievable from additional information

FROG measurements of DL fibre laser (Trina Ng)

Higher resolution through “X-FROG “ cross-correlation, frequency resolved optical gating

standard FROG ultrafast laser diagnostics

Auto-correlation, not cross correlationSingle shot requires more intensity than reasonable from EO material limitation

R&D goals • Develop XFROG with realistic EO intensities - signal/noise issues; non-degenerate wavelengths (?)• Develop & demonstrate retrieval algorithms

- including “spliced data”

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Pushing the time resolution of electro-optic diagnosticsElectro-optic Materials - Bandwidth of Coulomb to Optical conversion - EO efficiency

Single-shot optical characterisation - bandwidth of single-shot optical readout - single to noise - single-shot X-FROG development Practical diagnostic system issues Minimising laser requirements - Reliability, robustness“Non- invasiveness” - signal-noise, time resolutionFeedback or tune-up -repetition rate, absolute vs relative temporal info

Current R&D focusIn collaboration with CERN (CLIC project) & University of Dundee

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Overall Summary

o Electro-optic techniques available for different parameter regimes

o Proven capability for explicit temporal characterisation up to ~100 fs rms electron bunch structure

o Highest time resolution time-explicit techniques limited by - material properties

- optical pulse duration

- laser system robustness

o Multiple-crystal detectors & novel materials to be investigated

o “FROG-TD” will solve laser pulse duration limitation - amplified laser essential

- data-splicing procedure to be determined

o Spectral-upconversion offers solution for feedback - with multiple-crystal arrangement

S.P. Jamison, Daresbury Injector Workshop, June 30, 2011